In the oil and gas industry, acoustic tools are used to provide measurements of the attributes (such as slowness, attenuation, coherent energy, instantaneous frequency) of various types of waves propagated from transmitter to receiver. These attributes are analyzed to determine, among other things, the rate of flow of a hydrocarbon (gas or oil) out of a producing borehole in the hydrocarbon production industry. This critical information fundamentally depends on permeability of the formation, viscosity of the hydrocarbon and the existence of fractures. Collecting and recording this information on a delayed or real time basis is known as well logging
Evaluation of physical properties such as pressure, temperature and wellbore trajectory in three-dimensional space and other borehole characteristics while extending a wellbore is known as measurements-while-drilling (MWD) and is standard practice in many drilling operations. MWD tools that measure formation parameters such as resistivity, porosity, sonic velocity, gamma ray, etc. of a formation are known as logging-while-drilling (LWD) tools. An essential formation parameter for determination in a drilling operation is the existence of gas deposits or zones in a formation, on a real time basis. Similarly, early detection of kick is essential information for conducting safe and efficient drilling operations.
For the above and other reasons, the oil industry has developed acoustic well logging techniques that involve placing an acoustic tool within a well bore to make measurements indicative of formation attributes such as compressional slowness (DTc), shear slowness (DTs) and Stoneley slowness (DTst). Sonic logs can be used as direct indications of subsurface properties and in combination with other logs and knowledge of subsurface properties can be used to determine subsurface parameters, such as those related to borehole structure stability, that can not be measured directly. Early efforts in this connection were reported by Rosenbaum in “Synthetic Microseismograms: Logging in Porous Formations”, Geophysics, Vol. 39, No. 1, (February 1974) the disclosure of which is incorporated by reference as though set forth at length.
Acoustic logging tools typically include a transmitter and an array of axially spaced acoustic detectors or receivers. These tools are operable to detect, as examples, formation compressional waves (P), formation shear waves (S) and Stoneley (St) waves. These measurements can be performed following drilling or intermediate drill string trips by wireline logging operations. In wireline logging, sonic monopole tools can be used to measure compression waves (P) and shear waves (S) in fast formations. In addition to wireline logging, techniques have been developed where piezoelectric transmitters and hydrophone receivers are imbedded within the walls of drill string segments so that sonic LWD operations can be performed.
Early LWD and sonic data processing techniques developed by the Schlumberger Technology Corporation such as a slowness-time-coherence (STC) method is disclosed in U.S. Pat. No. 4,594,691 to Kimball et al. entitled “Sonic Well Logging” as well as in Kimball et al. “Semblance Processing of Borehole Acoustic Array Data,” Geophysics, Vol. 49, No. 3 (March 1984). This method is most useful for non-dispersive waveforms (e.g. monopole compressional and shear head waves). For processing dispersive waveforms a dispersive slowness-time-coherence (DSTC) method is preferred. This process is disclosed in U.S. Pat. No. 5,278,805 to Kimball entitled “Sonic Well Logging Methods and Apparatus Utilizing Dispersive Wave Processing.” The disclosures of these patents, of common assignment with the subject application, as well as the noted Geophysics publication authored by an employee of Schlumberger are hereby also incorporated by reference.
Sonic wireline tools, such as a Dipole Shear Sonic Imager (DSI—trademark of Schlumberger) and Schlumberger's Sonic Scanner generally have a multi-pole source. A multi-pole source may include monopole, dipole and quadrupole modes of excitation. The monopole mode of excitation is used traditionally to generate compressional and shear head waves such that formation compressional and shear slowness logs can be obtained by processing the head wave components. The head wave components are non-dispersive and are generally processed by slowness-time-coherence (STC) methods as discussed in the Schlumberger Kimball et al. '691 patent and Vol. 49 Geophysics article noted above.
The slowness-time-coherence (STC) method is employed to process the monopole wireline or LWD sonic waveform signals for coherent arrivals, including the formation compressional, shear and borehole Stoneley waves. This method systematically computes the coherence (C) of the signals in time windows which start at a given time (T) and have a given window move-out slowness (S) across the array. The 2D plane C(S,T) is called the slowness-time-plane (STP). All the coherent arrivals in the waveform will show up in the STP as prominent coherent peaks. The compressional, shear and Stoneley slowness (DTc, DTs, and DTst) will be derived from the attributes of these coherent peaks.
Traditionally, the attributes associated with the wave components found in the STP are the slowness, time and the peak coherence values. These three attributes are used in a labeling algorithm, discussed below, to determine the compressional, shear and Stoneley slowness from all of the STP peak candidates. These attributes can also be used for quality control purposes.
Although determining traditional attributes has been highly effective in the past, a need exists for enhancing information with respect to well cementing operations at each diameter transition stage and at other irregular zones where full cement bonding around a casing may be lacking to insure the cementing integrity of the well and correct for improper cement estimates on a real time basis while it is efficient to perform a corrective cement squeeze operation.